Manages long-running operations with an API service.
When an API method normally takes long time to complete, it can be designed
to return [Operation][google.longrunning.Operation] to the client, and the client can use this
interface to receive the real response asynchronously by polling the
operation resource, or using `google.watcher.v1.Watcher` interface to watch
the response, or pass the operation resource to another API (such as Google
Cloud Pub/Sub API) to receive the response. Any API service that returns
long-running operations should implement the `Operations` interface so
developers can have a consistent client experience.